There are various devices for measuring the blood pressure of subjects in a continuous and non-invasive manner. Some such devices exploit the fact that the pulse transit time (or speed of propagation of heart pulse waves, or pulse wave velocity) varies with blood pressure (and a number of other factors). If all other factors are equal then one can track changes in blood pressure by monitoring the pulse transit time or the speed at which pressure pulses propagate. One device which exploits this principle is described in U.S. Pat. No. 6,599,251 entitled CONTINUOUS NON-INVASIVE BLOOD PRESSURE MONITORING METHOD AND APPARATUS, which is hereby incorporated herein by reference. This device determines blood pressure based on a differential pulse transit time (DPTT) which is measured between two pulse waveforms obtained from two pulse signal detectors, for example optical pulse oximetry sensors, at two peripheral sites on a subject. DPTT is inversely proportional to the pulse propagation speed or pulse wave velocity. An increasing DPTT reflects a decreasing pulse propagation speed and an increasing blood pressure, and vice versa.
One issue with such CNIBP devices is that there are a large number of interacting factors which can effect the pulse propagation speed (or pulse wave velocity) or DPTT. These factors include:                the effects of vasoactive drug administration which alters the compliance of the arteries,        changes in the subject's physical position,        changes of sensor locations,        changes in the subject's blood density, and so on.        
Because the condition of a subject can vary over time the relationship between blood pressure and pulse propagation speed (or pulse wave velocity) or DPTT can also vary over time. It is therefore necessary to recalibrate blood pressure measurement systems which rely on pulse propagation speed (or pulse wave velocity) or DPTT measurements. Recalibration involves taking one or more reference blood pressure measurements. Reference blood pressure measurements may be obtained by way of any suitable alternative blood pressure measurement system such as a cuff-based system, an oscillometric blood pressure measurement mechanism, or the like. The reference blood pressure measurements can be used to redetermine coefficients in a CNIBP calculation formula which, for example, relates DPTT to blood pressure.
It is difficult to predict when the cumulative effect of factors which can affect the calculation that relates blood pressure to DPTT or pulse wave velocity of a CNIBP system is sufficient to require that the system be recalibrated. This is especially true in the environments in which such systems are often used. There is no simple rule for when to manually recalibrate a CNIBP system which takes into account all the effects that may lead to changes to the relationship between blood pressure and the pulse propagation speed (or pulse wave velocity) or DPTT. There is, therefore, a need for systems capable of reliably triggering recalibration when necessary.